1. Field of the Invention
This invention relates to a zoom lens, and particularly to a zoom lens suitable for a television camera, a phototaking camera, a video camera, etc. which appropriately uses an aspherical surface in a portion of a lens system, which has a large aperture in which the F number at the wide angle end is of the order of 1.7 and moreover has a wide angle (wide angle end angle of view 2.omega.=58.degree. to 70.degree.) and good optical performance over the entire variable power range of as high a variable power ratio as a variable power ratio of the order of 8.5 to 10.
2. Related Background Art
Zoom lenses having a large aperture, high variable power and high optical performance have heretofore been required for television cameras, phototaking cameras, video cameras, etc.
In addition, particularly in color television cameras for broadcasting, great importance has been attached to operability and mobility and in compliance with such requirements, compact CCD's (solid state image pickup devices) of 2/3 inch and 1/2 inch have become the mainstream among image pickup devices.
These CCD's have substantially uniform resolving power over the entire image pickup range and therefore, it has been required of zoom lenses using these that resolving power be substantially uniform from the center of the image field to the periphery of the image field.
For example, it is desired that various aberrations such as astigmatism, distortion and chromatic difference of magnification be corrected and the entire image field has high optical performance. It is further desired that the zoom lenses have a large aperture, a wide angle and a high variable power ratio and moreover be compact and light in weight and have a long back focus for disposing a color resolving optical system and various filters in front of image pickup means.
Among zoom lenses, so-called four-unit zoom lenses comprising, in succession from the object side, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for focal length change, a third lens unit of positive or negative refractive power for correcting the movement of an image plane fluctuating with a focal length change, and a fourth lens unit of positive refractive power performing chiefly the imaging action are often used as zoom lenses for color television cameras for broadcasting stations.
Among the zoom lenses of such four-unit construction, a four-unit zoom lens having F number of the order of 1.7, a wide angle end angle of view 2.omega.=86.degree., a great aperture ratio and high variable power of a variable power ratio of the order of 8 is proposed, for example, in Japanese Laid-Open Patent Application No. 6-242378.
In a zoom lens, to obtain a great aperture ratio (F number 1.7 to 1.8), a high variable power ratio (variable power ratio 8.5 to 10), a super-wide angle (wide angle end angle of view 2.omega.=90.degree. to 96.degree.) and moreover high optical performance over the entire variable power range, it is necessary to appropriately set the refractive power and lens construction of each lens unit.
Generally, to obtain a small aberration fluctuation and high optical performance over the entire variable power range, it becomes necessary, for example, to increase the number of lenses in each lens unit and increase the degree of freedom of design in aberration correction.
If for this purpose, an attempt is made to achieve a zoom lens of a great aperture ratio, a super-wide angle and a high variable power ratio, the number of lenses will unavoidably be increased, and this leads to the arising of the problem that the entire lens system becomes bulky, and it becomes impossible to comply with the desire for compactness and lighter weight.
Also, in the imaging performance, first speaking regarding the super-wide angle of a zoom lens, distortion poses the greatest problem. This is because distortion influences by the cube of the angle of view in the area of the third-order aberration coefficient.
As shown in FIG. 29 of the accompanying drawings, distortion is considerably greater under (minus) at the wide angle end (focal length fw). From the wide angle end fw toward the telephoto end (focal length ft), distortion becomes gradually greater in the direction of over (plus) and passes a zoom position at which distortion is 0, and the value of over tends to become greatest near the zoom position fm=fw.times.Z.sup.1/4. From the focal length fm to the telephoto end ft, the over amount becomes gradually smaller. In the foregoing, fw is the focal length at the wide angle end, and Z is a zoom ratio.
This tendency comes to remarkably present itself as the angle of view at the wide angle end becomes greater. In such a super-wide angle zoom lens wherein the wide angle end angle of view 2.omega. exceeds 90.degree., distortion of under on the wide angle side is created and the correction of this distortion becomes very difficult.
Next, the changing of a point at which the image contrast of the center of the image field is best, i.e., the so-called best imaging plane, resulting from a focal length change, poses a problem. This is attributable chiefly to the changing of spherical aberration resulting from a focal length change. This spherical aberration influences by the cube of the aperture in the area of the third-order aberration coefficient and therefore, it is the greatest problem in providing a large aperture.
Generally, the changing of spherical aberration resulting from a focal length change tends to be under (minus) with respect to the Gaussian imaging plane from the wide angle end at which spherical aberration is 0 to the vicinity of the zoom position fm=fw.times.Z.sup.1/4 as shown in FIG. 30 of the accompanying drawings when the zoom ratio is Z and the focal length at the wide angle end is fw. When the vicinity of the zoom position fm=fw.times.Z.sup.1/4 is passed, the under amount becomes smaller and becomes 0 at a certain zoom position, and now tends to become over (plus).
It becomes most over (plus) near a zoom position fd=(Fno.w/Fno.t).times.ft at which F drop in which F number becomes great (the lens system becomes dark) begins, and when this zoom position is passed, the over amount becomes smaller to the telephoto end and becomes substantially 0 at the telephoto end.
In the foregoing, Fno.w and Fno.t are F numbers at the wide angle end and the telephoto end, respectively, and ft is the focal length at the telephoto end.
As described above, particularly in a zoom lens having a position at which F drop begins, the correction of spherical aberration on the telephoto side becomes very difficult.
In order to correct such changing of various aberrations effectively over the entire variable power range, the number of lenses in the focusing lens unit and the lens unit of the focal length changing system has heretofore been increased. Such a technique, however, gives rise to a new problem that the entire lens system becomes bulky and complicated.
Also, the introduction of an aspherical surface for the solution of such a problem is done in an embodiment disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 6-242378.
However, the specification of zoom lenses has been improved, and in a zoom lens of a great aperture ratio and moreover, a high variable power ratio beginning from a super-wide angle, the reconsideration of the method of introducing the aspherical surface has become necessary.
In a zoom lens of a great aperture ratio and moreover a high variable power ratio beginning from a super-wide angle, distortion changes greatly on the wide angle side and spherical aberration changes greatly on the telephoto side. It has become difficult to correct these two aberrations efficiently and well simply by introducing an aspherical surface into one of the surfaces of a focal length changing portion.